The present invention generally relates to an atrial defibrillator and method for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved atrial sensing by interrupting the detection of atrial activity of the heart when a ventricular activation (R wave) of the heart is detected.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has precluded these defibrillators from becoming a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved operation are fully described in copending U.S. application Ser. No. 07/685,130, filed Apr. 12, 1991, in the names of John M. Adams and Clifton A. Alferness for IMPROVED ATRIAL DEFIBRILLATOR AND METHOD and U.S. application Ser. No. 07/856,514, filed Mar. 24, 1992, in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen for IMPROVED ATRIAL DEFIBRILLATOR, LEAD SYSTEMS, AND METHOD, which applications are assigned to the assignee of the present invention and incorporated herein by reference. As disclosed in the aforementioned referenced applications, synchronizing the delivery of the defibrillatinq or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart has been considered important to avoid inducing ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. The atrial defibrillators of the aforementioned referenced applications exhibit improved safety from inducing ventricular fibrillation by sensing ventricular activations of the heart in a manner which avoids detecting noise as ventricular electrical activations for generating reliable synchronization signals. Hence, these implantable atrial defibrillators, by providing such noise immunity in R wave detection assure reliable synchronization.
It is equally as important for an atrial defibrillator to reliably sense atrial activity. It is the atrial activity of the heart sensed by an automatic implantable atrial defibrillator which is utilized by the defibrillator to determine if atrial fibrillation is present and hence if the atria of the heart are in need of cardioversion. The atrial defibrillator of the aforementioned referenced applications include atrial electrodes for both applying cardioverting electrical energy to the atria and for enabling the sensing of atrial activity of the heart. One electrode is placed in the right atrium and another electrode is fed into the coronary sinus to a position near the left ventricle adjacent the left atrium. This electrode placement confines substantially all of the cardioverting electrical energy to the atria of the heart. The foregoing electrode placement also permits bipolar sensing of the activity of the atria.
In accordance with the referenced copending applications, the atrial defibrillators disclosed therein continuously detect ventricular activations of the heart. The ventricular activations are recorded and analyzed and when the ventricular activation rate indicates the potential of atrial fibrillation, the sensing of atrial activity is enabled along with an atrial fibrillation detector. The sensed atrial activity is digitized into digital samples which are recorded and then analyzed by the atrial fibrillation detector. The atrial fibrillation detector is implemented by a microprocessor which processes the recorded digital samples in accordance with a stored algorithm. If it is determined that the atria are in fibrillation and thus in need of cardioversion, the microprocessor causes cardioverting electrical energy to be stored in a storage capacitor which, at an appropriate time, is discharged into the atrial electrodes for cardioverting the atria.
In processing the atrial activity digital samples to determine if the atria are in fibrillation, it is most desirable for the digital samples processed by the atrial fibrillation detector to be representative of only the activity of the atria. However, since the ventricular activations are the most prominent electrical signals produced by the heart, the ventricular activations may also be sensed by the atrial electrodes along with the atrial activity. This becomes more likely if the atrial electrodes are widely spaced apart and/or close to at least one of the ventricles of the heart.
The atrial defibrillator and method of the present invention provides improved atrial activity sensing by interrupting the detection of atrial activity of the heart when ventricular activations (R waves) of the heart are detected. This assures that the atrial fibrillation detector will process only digital samples of atrial activity when detecting for atrial fibrillation. As disclosed herein in connection with the preferred embodiment in accordance with the present invention, the interruption in atrial activity detection may be accomplished by disabling the atrial sense amplifier coupled to the atrial electrodes during ventricular activations and/or by the microprocessor precluding the recording of atrial activity during ventricular activations. With either or both implementations, it will be assured that the atrial fibrillation determination is based solely upon atrial activity of the heart.